There is known a conventional technique that detects an object such as a person or item in a vehicle. Patent Literature 1 proposes a technique that transmits a microwave toward a vehicle in a parking lot, receives a reflected wave reflected from inside the vehicle by a device provided in the parking lot, and detects the presence or absence of a person inside the vehicle on the basis of the reflected wave being received.
However, a portion of the transmitted radio wave reflected from a human body inside the vehicle is smaller than a portion of the transmitted radio wave absorbed by the human body. That is, the above technique detects the presence or absence of an object inside the vehicle on the basis of the reflected wave with weak radio wave intensity, and can thus have reduced detection accuracy.